Search by light or electron microscopy among samples acquired by space probes, or photographs in situ, may show objects that seem candidates for classification as living organisms. Those closely resembling known terrestrial forms will be suspect as contaminants, but repeated occurrence in controlled samples would demand further investigation. Because our own microbial population is still imperfectly known and morphologically recognizable objects of unknown nature still turn up on this planet, a modern comprehensive Atlas of microscopic objects is urgently needed. The decision on whether a microscopic object is a candidate is, it is suggested, largely aesthetic. Presence of a bounding membrane, complex internal sub-microscopic structure, some degree of symmetry and, within a species, some uniformity of size seems essential. Certain patterns recur: but this in turn can be misleading as illustrated by a series of terrestrial polyhedral objects, and by growth of whiskers from monomer vapour. Confirmation of candidates selected must come from evidence that they are active and interact with the ambient environment (which could be a culture dish or inoculated animal or plant). Risks involved in bringing material back to Earth must be stressed.